Iran's Revolutionary Guard says it has seized a British oil tanker in the Strait of Hormuz.
The Government said it is "urgently" seeking information after reports a British-flagged oil tanker has been seized in the Strait of Hormuz near Iran.
The semi-official Fars news agency reported that the Iranian Revolutionary Guard had seized the Stena Impero.
A Government spokesman said: "We are urgently seeking further information and assessing the situation following reports of an incident in the Gulf."
A statement from Stena Bulk, the company which owns the tanker, said the firm had lost contact with the crew of 23 after it was approached by "unidentified small crafts and a helicopter" at around 4pm.
The statement said: "Stena Bulk and Northern Marine Management can confirm that at approximately 1600 BST on 19th July UK registered vessel Stena Impero (built 2018, 49,683 DWT) was approached by unidentified small crafts and a helicopter during transit of the Strait of Hormuz while the vessel was in international waters.
"We are presently unable to contact the vessel which is now heading north towards Iran.
"There are 23 seafarers aboard. There have been no reported injuries and their safety is of primary concern to both owners and managers.
"The priority of both vessel owner Stena Bulk and ship manager Northern Marine Management is the safety and welfare of the crew."
Foreign Secretary Jeremy Hunt said: "I'm extremely concerned by the seizure of two naval vessels by Iranian authorities in the Strait of Hormuz.
"I will shortly attend a COBR meeting to review what we know and what we can do to swiftly secure the release of the two vessels – a British-flagged vessel and a Liberian-flagged vessel.
"Their crews comprise a range of nationalities, but we understand there are no British citizens on board either ship.
"Our ambassador in Tehran is in contact with the Iranian Ministry of Foreign Affairs to resolve the situation and we are working closely with international partners.
"These seizures are unacceptable. It is essential that freedom of navigation is maintained and that all ships can move safely and freely in the region."
US President Donald Trump told reporters: "We will talk to the UK and we have no written agreement but we have an agreement. They've been a very great ally of ours.
"So we heard about it, we heard it was one, we heard it was two, and we will be working with the UK.
"They will have a new prime minister soon, which is a good thing, and we will be working with the UK – but we have no written agreement, but I think we have an agreement which is longstanding."